In accordance with the widespread use of portable electronic devices such as portable personal computers, handy video cameras and information terminals in recent years, non-aqueous electrolyte secondary batteries having a high voltage and a high energy density have been widely used as power sources. Furthermore, in view of environmental problems, battery automobiles and hybrid automobiles utilizing electrical power as a part of the power thereof have been put into practical use.
In non-aqueous electrolyte secondary batteries, various additives for non-aqueous electrolytes have been suggested so as to improve the stability and electric properties of the non-aqueous electrolyte secondary batteries. For example, it is considered that 1,3-propanesultone (for example, see Patent Literature 1), vinyl ethylene carbonate (for example, see Patent Literature 2), vinylene carbonate (for example, see Patent Literature 3), 1,3-propanesultone, butanesultone (for example, see Patent Literature 4), vinylene carbonate (for example, see Patent Literature 5), vinyl ethylene carbonate (for example, see Patent Literature 6) and the like form a stable film called an SEI (Solid Electrolyte Interface) on the surface of a negative electrode, and this film covers the surface of the negative electrode to suppress the reductive decomposition of a non-aqueous electrolyte. Furthermore, it is considered that a disiloxane having an unsaturated group such as a vinyl group (for example, see Patent Literature 7), a fluorosilane to which an alkenyl group is bonded (for example, see Patent Literature 8), an alkylenebisfluorosilane (for example, see Patent Literature 9), a fluorosilane to which an ether group is bonded (for example, see Patent Literature 10) and the like are adsorbed on the surface of a positive electrode to thereby protect the positive electrode and suppress the oxidative decomposition of a non-aqueous electrolyte.
On the other hand, it is known that 1,2-bis(difluoromethylsilyl)ethane can be used as an additive for lithium secondary batteries (for example, see Patent Literature 11), but any test result as a battery has not been disclosed, and any effect on positive electrode active materials has not been known at all.
Conventionally, lithium cobaltate has been widely used as a positive electrode active material in non-aqueous electrolyte secondary batteries, but the cost of cobalt as a raw material has been raising in recent years, and thus positive electrode active materials using inexpensive metal materials other than cobalt have been developed, and use of inexpensive positive electrodes using such positive electrode active materials has been rapidly prevailed. Polyanion compounds such as phosphate salt compounds containing a transition metal such as iron and lithium are excellent in performances in view of the output of a lithium secondary battery, but are inexpensive, whereas they have a problem that the elution of the transition metal such as iron easily occurs at a high temperature, and the capacity of the lithium secondary battery is decreased by repetitive use. Furthermore, lithium nickelates are excellent in performances from the viewpoints of the capacity and output of a lithium secondary battery. However, in general, an alkali component easily remains in a lithium-containing salt of a transition metal oxide containing much nickel, and thus a lithium secondary battery using a lithium nickelate as a positive electrode active material has a problem that the deterioration of a binder that is used in electrodes easily occurs by the residual alkali component and thus the durability of the lithium secondary battery becomes poor. However, the above-mentioned electrolyte additives that have been conventionally known could not exhibit a sufficiently advantageous effect on positive electrodes containing a polyanion compound and a lithium nickelate as positive electrode active materials, and thus further improvement has been demanded.